Project Summary/Abstract Hematogenously disseminated candidiasis is a major problem in hospitalized patients. Candida species are the third most commonly isolated organism from the bloodstream of these patients, and the attributable mortality of candidemia is 20% to 38%, despite currently available therapy. Candida albicans now causes less than 50% of cases of disseminated candidiasis, whereas the incidence of invasive infections caused by Candida glabrata is steadily increasing. The majority of C. glabrata isolates are fluconazole-resistant and a rising percentage are echinocandin-resistant. Therefore, new methods to prevent and treat disseminated candidiasis are urgently needed. During hematogenously disseminated candidiasis, blood-borne organisms must invade the endothelial cell lining of the vasculature to reach the target organs. Recently, we discovered that heat-labile serum proteins bind to the surface of yeast-phase C. albicans, C. glabrata, Candida parapsilosis, Candida tropicalis, and Candida krusei (collectively called Yeast-Phase Candida; YPC). Acting as bridging molecules, these serum proteins interact with receptors on the endothelial cell surface and induce endocytosis of the yeast. Based on these novel data, we hypothesize that the capacity of YPC to invade the endothelial cell lining of the vasculature is critical for the organism to escape from the bloodstream and establish a deep-seated infection. Our goal is to determine which endothelial cell receptors mediate the endocytosis of YPC, identify the serum proteins that act as bridging molecules and induce endocytosis, and define the fungal gene products that mediate binding to serum bridging molecules and serum-induced endocytosis. The experiments described in this proposal will identify and characterize a previously unexplored aspect of the host-pathogen interaction that plays a vital role in hematogenously disseminated infection by most medically important species of Candida. Utilizing this body of research, our ultimate goal is to develop new therapeutic strategies to block the escape of Candida spp. from the bloodstream. The use of bridging molecules to induce endothelial cell endocytosis is one of the first pathogenic mechanisms to be identified that is common to multiple species of Candida. Thus, interventions that inhibit this process hold promise to have broad therapeutic applicability.